If you don’t charge a lithium battery for a long time, it will eventually discharge and become unusable. A lithium battery will self-discharge at a rate of about 5% per month, so if you don’t use it for six months, the battery will be completely discharged. If you don’t charge a lithium battery for a long time, it will eventually die.
Lithium-ion batteries unavoidably degrade over time, beginning from the very first charge and continuing thereafter. However, while lithium-ion battery degradation is unavoidable, it is not unalterable. Rather, the rate at which lithium-ion batteries degrade during each cycle can vary significantly depending on the operating conditions.
That left less space for the ions to conduct charge, slowly degrading the battery. Rechargeable lithium-ion batteries don’t last forever. Over time, they hold onto less charge, eventually transforming from power sources to bricks. One reason: hidden, leaky hydrogen, new research suggests.
When people read “lithium battery”, most think of lithium-ion rechargeable, so called secondary cells. Hence both mine and Cristobols comments/answers. Your battery will degrade in storage, certainly significantly in 15 years. How much depends on conditions. The mechanisms of lithium-ion degradation are shown here.
If left unused for months, a fully charged lithium battery can become completely depleted. Capacity Loss: Over time, unused lithium batteries can lose their ability to hold a charge. This means that when you finally decide to use the battery, it might not last as long as it would have if it had been used regularly.
A battery that is only lightly discharged can often be recharged without any problems. However, if a battery is discharged below 2 volts per cell, it may be irreversibly damaged. It’s important to note that even if a lithium-ion battery is not being used, it will slowly self-discharge.